Separator for storage battery, storage battery and method of producing storage battery

ABSTRACT

The separator for storage battery of the present invention is a separator for storage battery mainly composed of microfibrous glass and an expanded microcapsule which has been kept in shape with its shell rendered water-permeable by expansion is incorporated in the aforesaid microfibrous glass so that an electrolyte can be retained in the gap between the glass fibers and in the expanded microcapsule to provide a high electrolyte retention and allow the aforesaid expanded microcapsule to act as a cushioning material, whereby the separator is provided with an enhanced restoring force under pressure and is thus kept the adhesion to the electrode over an extended period of time, making it possible to attain the enhancement of the storage battery capacity and the prolongation of its life and apply only a low pressure to incorporate the electrode group in the battery case during assembly of storage battery.

TECHNICAL FIELD

The present invention relates to a separator for storage battery mainlycomposed of microfibrous glass, a storage battery comprising such aseparator and a method of producing a storage battery.

BACKGROUND ART

A separator for storage battery such as a separator for valve regulatedlead-acid battery is required to act as an insulating material that isinterposed between a positive electrode and a negative electrode toseparate the two electrodes from each other and act to retain anelectrolyte. To this end, a storage battery is produced from a separatorfor storage battery mainly composed of ultrafine glass fibers which areexcellent in acid resistance, oxidation resistance and hydrophilicityand have an average fiber diameter of from about 0.6 to 2 μm capable ofincreasing porosity. In a storage battery comprising a separator mainlycomposed of such microfibrous glass having a high electrolyte retention,when an electrolyte is injected into the battery case, the frictionalforce of the glass fibers with each other is reduced to cause the glassfibers to move, weakening the sheet structure and hence causing thepressure of the separator against the electrode to drop from that of theinitial stage of assembly of storage battery. When the pressure runsshort, the adhesion between the separator and the electrodedeteriorates, making the reduction of battery capacity and lifeunavoidable. It is thus necessary in assembly of storage battery that anelectrode group comprising a separator interposed between electrodes bepreviously pressed before being incorporated in the battery case so thatthe pressure can be kept as high as possible even after the injection ofthe electrolyte, and this was disadvantageous in that the pressurerequired for incorporation of electrode group in battery is as high asfrom 49 to 98 kPa, troubling the battery assembly and hencedeteriorating productivity.

In order to cope with these problems, for example, JP-A-59-138059 andJP-A-7-122291 propose a storage battery which is arranged such that theelectrode is pressed by the expansion of a separator caused by heating,taking into account the fact that when subjected to repeated cycle ofcharge and discharge, the positive electrode of a storage batteryexpands and shrinks to undergo volumetric change that makes the bond ofparticles of active material constituting the positive electrode eachother relax, atomizes and softens the particles and causes the particlesto be exfoliative.

Referring to the storage batteries proposed in the above cited patentreferences, a valve regulated lead-acid battery is produced by puttingan electrode group comprising a separator interposed between a positiveelectrode and a negative electrode in a battery case, heating theassembly to allow the hollow body in the separator to expand so that apressure is applied to the positive electrode, and then injecting anelectrolyte into the battery case.

However, in the case where a separator as proposed in the above citedpatent references is used, it is certain that the problem of reductionof productivity of battery assembly due to the rise of the pressurerequired to incorporate the electrode group in battery as seen with theaforesaid conventional separator mainly composed of glass fibers aloneand storage battery can be solved, but the incorporation of a minutehollow body in the gap between the glass fibers which are a maincomponent of the separator causes the reduction of the porosity of theseparator and hence the deterioration of the electrolyte retention,i.e., wicking ability or wicking volume of the separator.

Therefore, an object of the present invention is to provide a separatorfor storage battery mainly composed of microfibrous glass capable ofenhancing productivity of battery assembly without deterioratingelectrolyte retention, a storage battery comprising the aforesaidseparator and a method of producing the same in order to eliminate thedisadvantages of the above cited patent references.

DISCLOSURE OF THE INVENTION

In order to accomplish the aforesaid object, in a first embodiment ofthe invention, the separator for storage battery of the presentinvention is a separator for storage battery mainly composed ofmicrofibrous glass, characterized in that an expandable microcapsule isincorporated in the microfibrous glass and then expanded, or anexpandable microcapsule is previously expanded and then incorporated inthe microfibrous glass, so that an expanded microcapsule kept in shapewith its shell rendered water-permeable by expansion is incorporated inthe microfibrous glass.

Further, in a second embodiment of the invention, a separator forstorage battery concerns a separator for storage battery of the firstembodiment, wherein the microcapsule is made of an acid-resistantthermoplastic resin.

Further, in a third embodiment of the invention, the separator forstorage battery concerns a separator for storage battery of the secondembodiment, wherein the microcapsule is made of apolyacrylonitrile-based resin.

In order to accomplish the aforesaid object, in a fourth embodiment ofthe invention, the separator for storage battery of the presentinvention is a separator for storage battery mainly composed ofmicrofibrous glass, characterized in that an expandable microcapsulewhich can be kept in shape while rendering its shell water-permeable byexpansion is incorporated in the microfibrous glass.

Further, in a fifth embodiment of the invention, the separator forstorage battery concerns a separator for storage battery of the fourthembodiment, wherein the microcapsule is made of an acid-resistantthermoplastic resin.

Further, in a sixth embodiment of the invention, the separator forstorage battery concerns a separator for storage battery of the fifthembodiment, wherein the microcapsule is made of apolyacrylonitrile-based resin.

Further, in order to accomplish the aforesaid object, the storagebattery of a seventh embodiment of the present invention comprises anelectrode group with a separator described in any one of the first tothird embodiments interposed between electrodes.

Further, in order to accomplish the aforesaid object, an eighthembodiment of the invention is a method of producing a storage batterycomprising disposing a separator mainly composed of microfibrous glasshaving an expandable microcapsule incorporated therein interposedbetween a positive electrode and a negative electrode to form anelectrode group, putting the electrode group in a battery case, and thenallowing the microcapsule to expand before or after the injection of anelectrolyte to render the shell water-permeable.

BEST MODE FOR CARRYING OUT THE INVENTION

Next, the separator for storage battery of the present invention, thestorage battery and the method of producing the same will be described.

As the microfibrous glass which is a main component of the separator,there is used, e.g., one having an average fiber diameter of from 0.2 to2 μm obtained by melting acid-resistant C glass, spinning the moltenglass, and then blowing the glass thus spun with an energy of burnerflame.

Further, as the unexpanded expandable microcapsule to be incorporated inthe aforesaid microfibrous glass, there is used one having a structurehaving an electrolyte resistance (acid resistance) and incorporating ina capsule, i.e., shell an expandable material which expands when it isheated or comes in contact with an electrolyte or otherwise, e.g., lowboiling hydrocarbon. Further, as the expandable material to beincorporated in the expandable microcapsule, there is selected one whichhas no adverse effects on the properties of the electrolyte, etc. evenwhen it leaks out of the capsule.

Further, the size of the unexpanded expandable microcapsule, is notgreater than several tens of micrometers as calculated in terms ofdiameter, taking into account the uniformity in dispersion during papermaking.

As the material of the shell of the aforesaid unexpanded expandablemicrocapsule or expanded microcapsule, there is selected one having anelectrolyte resistance which exhibits a strength such that it can keepin shape even after expansion. In particular, in the present invention,it is necessary that the shell of the expanded microcapsule be renderedwater-permeable, and in this case, it is necessary that the shell keepin shape while being rendered water-permeable. Examples of the materialthat can meet these requirements include expandable thermoplasticpolyethylene-based, polyolefin-based and polyacrylonitrile-basedmaterials, but polyacrylonitrile-based materials are preferred becausethey have an excellent acid resistance and little gas permeation thatallows the shell to fairly keep in shape.

The content of the aforesaid unexpanded expandable microcapsule orexpanded microcapsule preferably falls within a range of from 1 to 70%by weight for the sake of sheet formation. It is more desirableparticularly when the content is from 1 to 10% by weight. This isbecause when overheated, a thermoplastic microcapsule forms a film thatinhibits the hydrophilicity of the separator, possibly causing the riseof electrical resistance or the drop of wicking ability.

The separator of the present invention can be produced, e.g., by thefollowing method.

-   (1) To add a predetermined amount of an unexpanded expandable    microcapsule to a microfibrous glass as a main component and then    subject the mixture to uniform dispersion/mixing in water by a    separator such as mixer and pulper.-   (2) To add a proper amount of a cationic adsorbent, e.g.,    polyacrylamide-based adsorbent to the mixture thus uniformly    dispersed/mixed so that the expandable microcapsule is adsorbed to    and supported on the glass fibers.-   (3) To form the paper seed thus obtained into sheet by a cylinder    paper machine, wire paper machine or inclined paper machine.-   (4) To dry the sheet to obtain a raw sheet.

The raw sheet thus obtained can be incorporated as a separator in abattery, but in the case where an unexpanded expandable microcapsulewhich expands when it is heated is incorporated therein as it is, theexpandable microcapsule may be allowed to expand before beingincorporated in the battery while the shell thereof being renderedwater-permeable to obtain a separator incorporating a low densityexpanded microcapsule which is then incorporated in the battery. In thiscase, the high cushioning properties of the separator incorporating anexpanded microcapsule makes it possible to easily reduce the thicknessof the separator even under a low pressure and hence easily performbattery assembly without requiring a high pressure as in the relatedart.

On the other hand, in the case where the separator is incorporated inthe battery with the expandable microcapsule left unexpanded, theexpandable microcapsule may be allowed to expand after beingincorporated in the battery so that a predetermined pressure isobtained, making it unnecessary to apply any pressure during batteryassembly and hence making it extremely easy to assemble a battery. Theseparator thus incorporated in the battery is subjected to treatmentsuch as heating before the injection of the electrolyte so that theexpandable microcapsule expands to render the shell thereofwater-permeable, causing the thickness of the separator to rise andhence giving a predetermined pressure. In this case, the pressure can bearbitrarily adjusted by the expansion force and added amount of theexpandable microcapsule, the treatment temperature, the amount of theexpandable material to be incorporated, etc. However, the condition ofthe expanded microcapsule is needed to keep its shell water-permeable,but the shell must not be ruptured or destroyed. As the method ofallowing the expandable microcapsule to expand, there may be alsoproposed a method which comprises incorporating a reactive materialreactive with sulfuric acid in the electrolyte such as sodiumbicarbonate so that the expandable microcapsule expands at the same timewith or after the injection of the electrolyte, besides theaforementioned heating method. This method is advantageous in that theaforementioned heat treatment is not needed but is disadvantageous inthat it is necessary to inject the electrolyte that increases the amountof the sulfuric acid consumed for the reaction.

By the way, referring to the condition of the separator after theexpansion of the expandable microcapsule, it is desirable thatessentially the whole of the expanded microcapsules incorporated in theseparator keep in shape while its shell being rendered water-permeableby expansion, but since it is difficult to perform such a controlcompletely, for example the whole of the expanded microcapsules maycomprise some expanded microcapsules the shell of which have not beenrendered water-permeable even by expansion or some expandedmicrocapsules which have been ruptured or destroyed because they couldnot keep in shape after expansion. Further, referring to the waterpermeability given to the shell by the expansion of the expandablemicrocapsule, it is not necessarily required that the entire shell bewater-permeable, but there may be left some regions having no waterpermeability.

EXAMPLE

Next, specific examples of the present invention will be described indetail with related art examples, but the present invention is notlimited thereto.

Example 1

95% by weight of glass fibers having an average fiber diameter of 0.7 μmand 5% by weight of “Matsumoto Microsphere F-55” produced by MatsumotoYushi-Seiyaku Co., Ltd., as a thermally expandable microcapsule powdermade of polyacrylonitrile-based resin were dispersed and mixed in water.To the mixture was then added an acrylamide-based adsorbent so that theexpandable microcapsule was adsorbed to and supported on the glassfibers. Using an ordinary paper machine, the mixture was formed into asheet which was then dried at 95° C. to obtain a separator sheet.

Example 2

90% by weight of glass fibers having an average fiber diameter of 0.7 μmand 10% by weight of “Matsumoto Microsphere F-55” produced by MatsumotoYushi-Seiyaku Co., Ltd., as a thermally expandable microcapsule powdermade of polyacrylonitrile-based resin were dispersed and mixed in water.To the mixture was then added an acrylamide-based adsorbent so that theexpandable microcapsule was adsorbed to and supported on the glassfibers. Using an ordinary paper machine, the mixture was formed into asheet which was then dried at 95° C. to obtain a separator sheet.

Example 3

80% by weight of glass fibers having an average fiber diameter of 0.7 μmand 20% by weight of “Matsumoto Microsphere F-55” produced by MatsumotoYushi-Seiyaku Co., Ltd., as a thermally expandable microcapsule powdermade of polyacrylonitrile-based resin were dispersed and mixed in water.To the mixture was then added an acrylamide-based adsorbent so that theexpandable microcapsule was adsorbed to and supported on the glassfibers. Using an ordinary paper machine, the mixture was formed into asheet which was then dried at 95° C. to obtain a separator sheet.

Example 4

50% by weight of glass fibers having an average fiber diameter of 0.7 μmand 50% by weight of “Matsumoto Microsphere F-55” produced by MatsumotoYushi-Seiyaku Co., Ltd., as a thermally expandable microcapsule powdermade of polyacrylonitrile-based resin were dispersed and mixed in water.To the mixture was then added an acrylamide-based adsorbent so that theexpandable microcapsule was adsorbed to and supported on the glassfibers. Using an ordinary paper machine, the mixture was formed into asheet which was then dried at 95° C. to obtain a separator sheet.

Subsequently, the separators of Examples 1 to 4 thus obtained were eachincorporated in a 6M4 (abbreviation of 6V4Ah) battery under no pressure.Thereafter, the aforesaid battery was subjected to heat treatment at120° C. so that the expandable microcapsule in the separator was allowedto expand, rendering the shell of the expanded microcapsulewater-permeable while increasing the thickness of the separator to applya predetermined pressure to the electrode group. Thereafter, anelectrolyte was injected into the battery to obtain a valve regulatedlead-acid battery.

Conventional Example

100% by weight of glass fibers having an average fiber diameter of 0.7μm was dispersed in water, and then formed by an ordinary paper machineinto a sheet which was then dried at 95° C. to obtain a separator sheet.

Subsequently, the conventional separator thus obtained was incorporatedin a 6M4 battery under an initial pressure of 19.6 kPa. Thereafter, anelectrolyte was injected into the battery to obtain a valve regulatedlead-acid battery.

Subsequently, the separators of Examples 1 to 4 and conventional Examplethus obtained and the batteries comprising these separators weremeasured for separator properties and battery properties. The resultsare set forth in Table 1.

TABLE 1 Conventional Item Unit Example 1 Example 2 Example 3 Example 4Example 1 Material Glass fibers (0.7 μm) wt-% 95 90 80 50 100compounding Microcapsule wt-% 5 10 20 50 — Material cost — 80 80 80 80100 Separator Initial thickness mm 0.78 0.78 0.55 0.21 1.05 propertiesThickness after expanded mm 1.05 1.08 1.05 1.05 — Grammage g/m² 102 9568 55 150 Density g/cm³ 0.097 0.086 0.065 0.052 0.150 Maximum porediameter μm 19.0 19.0 20.5 23.0 19.1 Cushioning properties % 58 55 52 4570 Wicking volume % 92 93 93 94 88 Battery Battery Initial kPa 0 0 0 019.6 properties pressure During electrolyte kPa 20.2 21.6 19.6 19.6 11.8injection/expansion Cycle life — 120 120 130 130 100 Note 1) Cost:Relative to that of Conventional Example 1 as 100 Note 2) Cushioningproperties: (Thickness under pressure of 98 kPa/thickness under pressureof 19.6 kPa) × 100 Note 3) Cycle life: A cycle life test was conductedwith 2 hours of charge at 1 A and 6 hours of discharge at 0.4 A as onecycle. The lifetime was judged when the capacity fell below 50% of thenominal capacity after discharging to 5.1 V at 1 A. The figure indicatesthe value relative to that of Conventional Example 1 as 100. Note 4) Forthe evaluation of the separator properties of Examples 1 to 4 except theinitial thickness, the sheet after expansion was examined.

As can be seen in Table 1, the separators of Examples 1 to 4 areseparators comprising an expandable microcapsule incorporated thereinand thus can attain the reduction of density, making it possible toreduce the material cost by 20% from that of the conventional example.Further, the grammage could be reduced by from 32 to 63% from theconventional example, making it possible to attain the enhancement ofpaper making speed and the drastic reduction of drying energy duringproduction. Moreover, by rendering the shell of the expandablemicrocapsule water-permeable by expansion, the electrolyte can beretained also in the expanded microcapsule, making it possible to raisethe wicking volume by from 5 to 7% from that of the conventionalexample.

Further, when it is attempted to reduce the density by the materialconstitution of the conventional example, i.e., constitution comprisingglass fibers alone, there is formed a sheet having a coarse structureand hence raised maximum pore diameter that can easily cause thestratification of the electrolyte leading to the drop of the batterycapacity, that is, the reduction of the life of the battery, but theseparators of Examples 1 to 4 have the gap between fibers filled withexpanded microcapsules, making it possible to inhibit the rise of porediameter as much as possible despite its reduced density.

Moreover, the batteries comprising the separators of Examples 1 to 4showed a cycle life rise of from 20 to 30% as compared with the batterycomprising the separator of the conventional example. This is presumablybecause the battery comprising the separator of the conventional examplepossesses an initial battery pressure of 19.6 kPa but exhibits a drop to11.8 pKa after the injection of the electrolyte and a low separatorrestoring force after pressurized and thus gradually loses adhesion tothe electrode with time and reduces its life. On the contrary, in thebatteries comprising the separators of Examples 1 to 4, the expandedmicrocapsule acts as a cushioning material to provide the separator witha high restoring force under pressure, causing little or no pressuredrop after the injection of the electrolyte, making it possible tomaintain the adhesion to the electrode over an extended period of timeand hence resulting in the prolongation of life.

By the way, when a separator sheet comprising an expanded microcapsuleincorporated therein which is kept in shape with its shell renderedwater-permeable was produced from the same materials as used in theaforesaid examples by previously allowing an expandable microcapsule toexpand, and then incorporating the expanded microcapsule in microfibrousglass, the separator could be provided with a reduced density and anenhanced wicking volume and prevent itself from having an increased porediameter as in the aforesaid examples. Further, when a separator sheetcomprising an expanded microcapsule incorporated therein which is keptin shape with its shell rendered water-permeable was produced from thesame materials as used in the aforesaid examples by forming a sheetcomprising an expandable microcapsule incorporated in the aforesaidmicrofibrous glass, and then heating the aforesaid sheet so that theaforesaid expandable microcapsule is allowed to expand before beingincorporated in the battery, the separator could be provided with areduced density and an enhanced wicking volume and prevent itself fromhaving an increased pore diameter as in the aforesaid examples.Moreover, the valve regulated lead-acid battery produced byincorporating the aforesaid various separator sheets in the battery, andthen injecting the electrolyte in the battery could be provided with aprolonged cycle life as in the aforesaid examples.

INDUSTRIAL APPLICABILITY

In accordance with the separator for storage battery of the presentinvention, since the electrolyte can be retained in the gap between themicrofibrous glass which are a main component in the battery case andthe electrolyte can be absorbed also in the inside of the expandedmicrocapsule through the shell which has been rendered water-permeableby the expansion of the expandable microcapsule, a high electrolyteretention can be provided despite the structure comprising the gapbetween the fibers filled with expanded microcapsules as hollow body.

Further, in accordance with the separator for storage battery of thepresent invention, since the expanded microcapsule incorporated in theseparator can be kept in shape with its shell rendered water-permeableby expansion in the battery case, the expanded microcapsule acts as acushioning material to provide the separator with a high restoring forceunder pressure, causing little or no pressure drop after the injectionof the electrolyte and hence making it possible to maintain the adhesionto the electrode over an extended period of time and attain theenhancement of the storage battery capacity and the prolongation of itslife.

Moreover, since the electrode group can be incorporated in the batterycase under no or only a low pressure during assembly of storage battery,the battery can be easily assembled, enhancing the productivity.

Further, by incorporating an expandable microcapsule or expandedmicrocapsule in microfibrous glass, the amount of expensive microfibrousglass to be used in the separator can be reduced, making it possible toreduce the production cost.

1. A separator for storage battery mainly composed of microfibrous glassof average fiber diameter 0.2 to 2 μm, characterized in that anexpandable microcapsule is incorporated in said microfibrous glass andthen expanded, or an expandable microcapsule is previously expanded andthen incorporated in said microfibrous glass, so that an expandedmicrocapsule kept in shape with its shell rendered water-permeable byexpansion is incorporated in said microfibrous glass.
 2. The separatorfor storage battery as described in claim 1, wherein said microcapsuleis made of an acid-resistant thermoplastic resin.
 3. The separator forstorage battery as described in claim 2, wherein said microcapsule ismade of a polyacrylonitrile-based resin.
 4. A separator for storagebattery mainly composed of microfibrous glass of average fiber diameter0.2 to 2 μm, characterized in that an expandable microcapsule which canbe kept in shape while rendering its shell water-permeable by expansionis incorporated in said microfibrous glass.
 5. The separator for storagebattery as described in claim 4, wherein said microcapsule is made of anacid-resistant thermoplastic resin.
 6. The separator for storage batteryas described in claim 5, wherein said microcapsule is made of apolyacrylonitrile-based resin.
 7. A storage battery comprising anelectrode group with a separator interposed between a positive electrodeand a negative electrode, wherein said separator for storage battery ismainly composed of microfibrous glass of average fiber diameter 0.2 to 2μm, and is characterized in that an expandable microcapsule isincorporated in said microfibrous glass and then expanded, or anexpandable microcapsule is previously expanded and then incorporated insaid microfibrous glass, so that an expanded microcapsule kept in shapewith its shell rendered water-permeable by expansion is incorporated insaid microfibrous glass.
 8. A method of producing a storage batterywhich comprises disposing a separator mainly composed of microfibrousglass of average fiber diameter 0.2 to 2 μm having an expandablemicrocapsule incorporated therein interposed between a positiveelectrode and a negative electrode to form an electrode group, puttingsaid electrode group in a battery case, and then allowing saidexpandable microcapsule to expand before or after the injection of anelectrolyte to render the shell water-permeable.
 9. The separator forstorage battery of claim 1, wherein the content of said expandablemicrocapsule in the separator is 1 to 70% by weight.
 10. The separatorfor storage battery of claim 9, wherein the content of said expandablemicrocapsule in the separator is 1 to 10% by weight.
 11. The storagebattery of claim 7, wherein said microcapsule is made of anacid-resistant thermoplastic resin.
 12. The storage battery of claim 7,wherein said microcapsule is made of a polyacrylonitrile-based resin.